A conventional active noise reducing device, well suited particularly for vehicles, employs an adaptation feed-forward control method using an adaptive notch filter for reducing unpleasant muffled engine sound generated in a vehicle interior accompanying the driving of an engine. This conventional device includes a residual signal detector having a microphone rigidly mounted in the interior, a secondary noise generator having a speaker rigidly mounted also in the interior. The secondary noise generator placed permanently at the same location as the residual signal detector is combined with the detector in order to reduce the subject noise collected at the location of the detector. This prior art is disclosed in, e.g. Unexamined Japanese Patent Publication No. 2000-99037.
However, in the environment of a limited space of the interior, deep dips or sharp peaks sometimes occur in the gain characteristics of transmission from the secondary noise generator including the speaker to the residual signal detector including the microphone. These dips and peaks are caused by interference or reflection of sound-wave in the small interior space, and they are generated regardless of the locations of the residual signal detector and the secondary noise generator. The active noise reducing device in accordance with the prior art employs the secondary noise generator placed permanently at the same place as the residual signal detector for reducing the subject noise detected at the place of the residual signal detector. Thus there is great possibility that dips and peaks occur in the gain characteristics of the transmission from the secondary noise generator to the residual signal detector within the frequency band to which the noise reduction control is desirably applied. Within the frequency band where the dips and peaks occur, the transmission phase characteristics also changes sharply and the occurrence frequency per se has a great dispersion. The noise reduction control to be carried out in such a frequency band tends to invite unstable operation of the adaptive filter, so that ideal noise-reduction effect cannot be expected. In the worst case, the adaptive filter falls in divergent state and generates abnormal sound. On top of that, in such a frequency band, the secondary noise generated by the secondary noise generator is hard to reach to the residual signal detector, so that an output from the active noise reducing device increases and the secondary noise generator produces distorted sound.